Stay with your mp3's, converting them will sound like dung any way you do it. AAC should only be used if you're ripping from the CD. Give it a try for yourself, convert a couple mp3's>aac or mp3>wav>aac and stick on some good phones and have a listen.

Both MP3 and AAC are lossy encoders, meaning it drops information from the sound file that you probably won't notice to reduce filesize. On top of that, they encode differently so what what is choosen to be dropped is different between encoders. So when you convert from one lossy format to another you lose even more information. You should only convert to lossy formats from a lossless one, like from WAV to MP3 or from AIFF to AAC.

what if there's no choice. I have most of my music in WMA recorded at a very high bit rate becaseu I've been using Media Center 10. Don't ask me why but I just like I tunes better, the interface, the store, and the Party DJ option. So I have to convert my files. I think I want to go to 192 AAC which sounds good to me.